ARB is about to be launched, and L2 (Rollup) has also received more attention. Recently, there have been more and more debates about the advantages and disadvantages of the two solutions, OP and ZK. Some people say that OP is orthodox, and some people say that ZK is safer. This article will explore two popular second-layer expansion solutions in depth, as well as the recently highly anticipated Arbitrum. This is a technology-oriented article. I will focus on their working principles and characteristics, and explain them in plain language to help everyone better understand and evaluate these expansion solutions.
1. Overview of the expansion plan
With the rapid development of blockchain technology, scalability issues have become one of the main obstacles to the widespread application of blockchain. In order to increase network throughput and reduce transaction fees, capacity expansion is a must. Capacity expansion solutions are usually divided into two categories: one is off-chain capacity expansion (Layer 2), which is achieved by building a new protocol layer on top of the underlying blockchain; the other is on-chain capacity expansion (Layer 1), which improves throughput by optimizing the underlying blockchain protocol itself.
On-chain expansion solutions can be divided into sharding, choosing a more efficient consensus algorithm, and protocol optimization. Sharding is to divide the blockchain network into multiple independent sub-chains, each of which can process transactions in parallel. In this way, the throughput of the entire network will increase linearly with the increase in the number of sub-chains. Sharding is also a key step in the Ethereum 2.0 roadmap. After sharding, TPS and Gas can be truly optimized. In recent years, innovations in consensus algorithms have been rare. For example, the previously proposed POS, DPOS, DAG, etc. are all innovations relative to POW. Compared with POW, they can reduce network resource consumption and increase transaction processing speed. Similarly, Ethereum has chosen this path. The third solution is to optimize the underlying blockchain protocol, such as adjusting the block size, block generation time, etc., which can improve the network throughput to a certain extent, such as Bitcoin's Segregated Witness (SegWit) upgrade.
Off-chain expansion solutions can be divided into state channels, Plasma, and Rollups. State channels allow users to conduct transactions off-chain and interact with the main chain only when the channel is opened and closed, which greatly reduces the number of on-chain transactions and thus improves throughput. Raiden Network and Lightning Network are state channel expansion products for Ethereum and Bitcoin respectively. Plasma is a sidechain solution that allows users to migrate assets from the main chain to the sidechain and conduct transactions on the sidechain. The sidechain periodically submits its state updates to the mainchain to ensure security, such as OMG Network. Rollups are to package multiple transactions into a single proof (fraud proof of zk-SNARKs or Optimistic Rollup) and submit it to the mainchain. In this way, the mainchain only needs to verify the proof without processing each transaction, thereby improving throughput. Typical examples are zkSync (based on zkRollup) and Optimism (based on Optimistic Rollup). Arbitrum is also a product based on OP.
2. Optimistic Rollups 和 zk-Rollups
2.1 zk-Rollups
zk-Rollups is a second-layer expansion solution based on zero-knowledge proof. First, the Rollup Operator component aggregates multiple off-chain transactions into a batch, and then uses zero-knowledge proofs (such as zk-SNARKs or zk-STARKs) to generate a concise proof file, which can verify the validity of the entire batch of transactions without having to check each transaction one by one; then the proof and the data related to the batch are submitted to the main chain, which verifies the correctness of the proof to ensure that the transaction is valid; after the main chain verification is passed, the on-chain contract will update the on-chain status according to the data in the proof. This means that although the transaction is conducted off-chain, the on-chain status is still updated to ensure data consistency.
Note: Zero-Knowledge Proof (ZKP) is a cryptographic concept that allows a prover to prove to a verifier that a statement is true without revealing any other information about the statement. In short, zero-knowledge proof allows a person to prove that he has certain information without revealing the information itself.
2.2 Optimistic Rollups
Optimistic Rollups is a second-layer expansion solution based on optimistic verification, that is, the submitted blocks are correct by default unless someone questions it. It also requires the Rollup Operator to aggregate many off-chain transactions into a batch, and then calculate the new status (such as balance, contract status, etc.) generated by the batch transactions and generate an off-chain status update; then submit the off-chain status update and related data to the main chain. This status is correct by default and does not require additional verification; however, after the status update is submitted, there will be a fixed challenge period, during which anyone can question the validity of the submitted status update by providing a fraud proof. The entire transaction related to the questioned status will be run through the EVM for verification. If the status update is proven to be wrong, the submitter will be punished (deposit deducted), and the on-chain status will be rolled back to the correct state; if no one questions the status update during the challenge period, or the question is proven to be wrong, then the on-chain status will be updated according to the submitted status update.
2.3 Comparison between ZK and OP
ZK and OP each have their own characteristics. I analyze them from the following five different perspectives so that you can judge them according to your own inclinations:
1. Transaction verification method:
OP: Verify transactions through fraud proofs. OP assumes that transactions are valid by default unless someone submits evidence that a transaction is invalid. This requires continuous monitoring by off-chain users and nodes to ensure that the Rollup Operator is not doing evil.
ZK: Verify transactions through zero-knowledge proofs such as zk-SNARKs or zk-STARKs. ZK generates a concise proof to ensure the validity of transactions in a batch without having to check each transaction one by one.
2. Security:
OP: Since the default assumption is that transactions are valid, there may be certain security risks, and off-chain users and nodes need to actively monitor transactions to ensure security.
ZK: Zero-knowledge proof-based verification provides ZK with higher security because it requires generating a proof to ensure the validity of the transaction.
3. Throughput and performance:
OP: OP generally has faster off-chain transaction processing speed compared to ZK, but on-chain verification may take longer due to the need to wait for the challenge cycle of fraud proofs.
ZK: Although generating zero-knowledge proofs requires a certain amount of computing resources, ZK’s on-chain verification is faster because once the proof is generated, the main chain can quickly verify it.
4. Versatility
OP: OP is fully compatible with EVM, and many DAPPs can be directly migrated. The overall computational complexity of the solution is low, and it is more suitable for general smart contract execution and complex calculations.
ZK: Although zero-knowledge proof technology is under development, its application in general smart contracts and complex computing is currently subject to certain limitations.
5. Cost:
OP: Typically has lower off-chain transaction costs.
ZK: Generating zero-knowledge proofs requires certain computing resources, which may result in higher off-chain transaction costs.
In general, Optimistic Rollups and zk-Rollups have their own advantages and disadvantages. Optimistic Rollups are more suitable for handling complex smart contract scenarios and have better Ethereum compatibility; while zk-Rollups have advantages in security and privacy protection.
3. Decision
Arbitrum is a second-layer scaling solution based on Optimistic Rollups. It combines the advantages of Optimistic Rollups and innovates and optimizes the arbitration process. It uses binary search technology when handling questions and arbitration, reducing the complexity and cost of the arbitration process.
As mentioned above, in an optimistic verification system, when someone questions a submitted block, the arbitration process is initiated. The questioner needs to point out a specific error in the block, such as incorrect transaction execution results, incorrect status update, etc.
In order to efficiently find the location of the error, binary search divides the possible range of the error into two halves. The questioner needs to choose which half the error appears in and continue to search downward. For example, if the questioner believes that the error appears in the first half of the block, then they need to provide proof of the state update of that part; in each iteration, the questioner and the verifier continue to divide the possible range of the error into two halves. The questioner needs to point out which half the error appears in each iteration and provide corresponding proof, and the verifier needs to provide corresponding rebuttal evidence; through continuous binary search iterations, the questioner and the verifier gradually narrow the possible range of the error; eventually, when the range is narrowed to a specific transaction or state update, the questioner needs to provide detailed evidence to prove the existence of the error, and the verifier needs to provide corresponding rebuttal evidence; after a series of binary search iterations and evidence exchange, if the questioner can successfully prove that there is an error in the block, then the block will be deemed invalid, and if the verifier can successfully refute the questioner's evidence, then the block will be deemed valid. In this process, the wrong party will lose the deposit, and the winning party may receive a reward.
In summary, binary search arbitration can narrow the scope of errors. This process is performed off-chain, and the chain only needs to verify the final disputed part, thereby reducing the transaction processing cost on the chain. However, this process also prolongs the processing time, so in the event of arbitration, Arbitrum is cheaper than Optimistic, but also slower.
4. Conclusion
Many competitors in the same field are focusing on technology and ecology. OP and ZK are somewhat like Aptos and SUI. The final winner will depend on the users' vote.